1. Field of the Invention
The present invention relates to a solid-state image pickup device having a photoelectric conversion element for converting light into an electrical signal and a method of manufacturing the same.
2. Related Background Art
In recent years, as solid-state image pickup devices have smaller size, higher density, and higher resolution, a decrease in light-receiving sensitivity due to a decrease in aperture area of a region which is not covered by a light-shielding portion on the light incoming side of a photoelectric conversion element or a decrease in light-receiving area of the photoelectric conversion element poses a problem. To improve sensitivity, a lens that condenses incoming light is provided above the photoelectric conversion element to increase the aperture area of the solid-state image pickup device in effect.
A prior art of a solid-state image pickup device with such microlens will be explained below with reference to FIG. 1 in relation to Japanese Patent No. 2,558,389. Referring to FIG. 1, a photoelectric conversion element 102 is formed on a semiconductor substrate 101, and an insulating layer 103 is formed on the photoelectric conversion element 102 and the remaining surface of the semiconductor substrate 101. A polysilicon transfer electrode portion 104 for transferring photocharges of the photoelectric conversion element 102 is formed on the insulating layer 103 on the remaining surface of the semiconductor substrate 101, and an aluminum light-shielding portion 105 is formed on top of it. A surface passivation layer 106 is formed on the light-shielding portion 105, and a leveling layer 107 that consists of a transparent polymer resin and levels the element surface is formed thereon. Furthermore, a concave microlens layer 108 composed of a transparent polymer resin or materials such as casein, gelatin, and the like is formed, and an inter-lens layer 109 consisting of a transparent polymer resin is formed thereon. A round, convex microlens layer 110 consisting of a transparent polymer resin or casein, gelatin, and the like is formed on the interlayer 109, and a passivation layer 111 consisting of a transparent polymer resin is formed thereon.
With this structure, since the convex microlens layer 110 condenses light, sensitivity can be improved. Also, since the inter-lens layer 109 is interposed between the convex and concave microlens layers 110 and 108, the refraction index of the convex microlens layer 110 and its surface curvature required for focusing light to a size equivalent to that of an aperture portion on the concave microlens layer 108 can be reduced.
Let na, nb, nc, and nd be the refraction indices of the convex microlens layer 110, inter-lens layer 109, concave microlens layer 108, and leveling layer 107, respectively. If na&gt;nb, nc&gt;nb, and nc&gt;nd, i.e., (na, nc)&gt;(nb, nd), light can be condensed most efficiently and can enter the photoelectric conversion element nearly perpendicularly, thereby suppressing production of smear noise, and achieving a high S/N ratio.
Furthermore, since the leveling layer 107 sends light onto the surface of the photoelectric conversion element nearly perpendicularly and therefore the refraction index of the concave microlens and its surface curvature can be reduced, the device can be easily manufactured.
However, a microlens is used to assure an effective aperture ratio despite a small pixel size of the solid-state image pickup device of a photosensor, and the aperture ratio is improved by combining convex and concave lenses. This complicates the layer structure, resulting in high manufacturing cost and low manufacturing yield. Also, a plurality of alignment processes are required, and the effective aperture ratio cannot be desirably improved.
As a recent device has higher density, higher resolution, and smaller size, it is hard to match the optical axis of the microlens placed above the photoelectric conversion element as an underlayering device with the condensed point.